1. Field of the Invention
The present invention relates to a method for coating a paper web. The invention also concerns an apparatus for coating a paper web.
2. Description of the Prior Art
According to prior-art techniques, a coating mix layer is first metered onto a paper web using an applicator apparatus and the coat is next smoothed with the help of a doctor blade working against the backing roll of the coater station.
Particularly in coater stations equipped with a nozzle applicator, certain operating conditions have invoked runability problems of the web to be coated owe to the fact that the applicator apparatus and the actual doctor blade form two linear fixing points of the web onto a single backing roll. Air conveyed along with the surface texture of the paper web remains pocketed between the web and the backing roll in the area delimited by said two web fixing points. Then, an irregularly shaped air pouch called "blowing" can develop in the area between the web fixing points owe to such causes as, for instance, an uneven distribution of web tension. The forces exerting either a web braking or pulling effect at the web fixing points behave so as to allow web slackening in the area between the web fixing points. Furthermore, the web moisture content increases upon the application of the coating mix, whereby the web tends to swell between the web fixing points. This phenomenon causes additional slackening of the web.
Given an uneven tensile, thickness, porosity or other profile across the web, web slackening occurs irregularly across the web within the area between the web fixing points. This may cause curling in the machine direction, which in turn may result in creasing.